Notice the space after the full stop in "This is the start. ".
If you don’t put this space then your output will look like this This is the start.This is the end.,whereas the expected output is This is the start. This is the end.

If you are still can not pass the test after both of @sonimadhuri’s suggestions, then post your latest code into a reply.

When you enter a code block into the forum, precede it with a line of three backticks and follow it with a line of three backticks to make easier to read. See this post to find the backtick on your keyboard. The “preformatted text” tool in the editor (</>) will also add backticks around text.